Helicity scattering amplitudes in Quantum Field Theory (QFT) encode the full dependence
on the spin degrees of freedom of the particles involved in the scattering, and are the building blocks
for computing various kinds of physical observables through which we try to understand
the interactions among particles observed in nature.
The incorporation of spin degrees of freedom, or polarization effects, in terms of
spin- respectively polarization-dependent physical observables, leads to a richer phenomenology.
Such observables offer valuable means to discriminate different dynamical models,
in particular for discovering potential Beyond-Standard-Model effects.
For a review of the role of particle polarizations in testing the Standard Model and searching for new physics,
we refer to refs. [1]}, [2]}, [3]}, [4]}
and references therein.
